Apparent P/O ratio and chemical energy balance in frog sartorius muscle in vitro

We tested the hypothesis that there is a constant relation between the amount of chemical energy used during a single tetanus and the extent of subsequent aerobic recovery metabolism. Breakdown of high energy compounds (Δ P) during contraction was measured by rapid freezing techniques using isometric contractions of unpoisoned frog sartorius muscles at 0 °C. A stable and sensitive method was designed to study recovery O₂ consumption of similar contractions. In some cases, initial chemical changes and recovery O₂ consumption were measured in the same muscle. The ratio of these chemical changes yields a value for the P/O ratio of whole muscle. This ratio was found to be 2 and was constant for the range of contraction durations studied (5–20 s). Splitting of phosphorylcreatine or ATP after mechanical relaxation and glycolytic resynthesis of ATP could not be detected. Thus, the tested hypothesis is valid; but there is no ready explanation why this estimate of the P/O ratio of whole muscle differs from the ratio measured in isolated mitochondria. Because of this constancy and the observed stoichiometry unknown energy yielding reactions, postulated to occur either early or late in a maintained tetanus, are excluded by these experiments.